The present invention relates to an apparatus for laser annealing large-width substrates, said apparatus being formed from a plurality of juxtaposable laser modules, without particular limitation.
It is known to carry out local rapid laser annealing (laser flash heating) of coatings deposited on flat substrates. To do this, the substrate with the coating to be annealed is run under a laser line, or else a laser line is run over the substrate bearing the coating.
Laser annealing allows thin coatings to be heated to high temperatures, of about several hundred degrees, while preserving the underlying substrate. Run speeds are of course preferably as high as possible, advantageously at least several meters per minute.
The present invention in particular relates to lasers using laser diodes. The latter are currently the best laser sources from the point of view of price and power.
In order to obtain the powers per unit length required to implement a process with a high run speed, it is desirable to concentrate the radiation of a very large number of laser diodes into a single laser line. At the substrate bearing the coating to be annealed, the power density of this laser line must generally be as uniform as possible, so as to expose all the points of the substrate to the same annealing energy.
Moreover, it would be desirable to be able to treat, at high speed, substrates of large width, such as the “jumbo” size (6 m×3.21 m) flat glass sheets produced by float processes. The problem as regards obtaining very long laser lines lies with the production of a system allowing laser modules to be juxtaposed so that the length of the line may be increased at will while avoiding the need for monolithic optical assemblies that are as long as the line itself.
Such modular laser apparatuses have already been envisioned. Thus, prior art U.S. Pat. No. 6,717,105 describes a modular laser apparatus of very simple design. In this laser apparatus, each laser oscillator generates a laser beam that, after it has been shaped, is reflected by a mirror and projected, in the form of a laser line, at a right angle onto the substrate. The means for shaping the laser beam are designed so as to form, at the intersection with the substrate, a laser line having a top-hat power density profile with a very wide central portion in which the power density is high and constant and, at either end of this plateau, steeply sloped falling sides. The profiles are combined by superposing the steep sides so as to create a single combined line with a power density distribution that is as uniform as possible.
Laser modules generating this type of top-hat laser line profile are however very sensitive to possible errors made when positioning the modules relative to one another. Specifically, the high steepness of the slopes in the superposed lateral zones means that if the spacing between modules is too large a power density flaw is easily formed at the junction and, conversely, modules that are too closely spaced will lead to junction zones in which the power density is excessively high.
The Applicant has observed that it is possible to significantly decrease the sensitivity of a modular laser apparatus to errors in the positioning of respective laser modules by equipping the modules of the apparatus with a microlens-based, laser-line shaping, optical system that does not create a “top-hat” power density profile, as was generally the case in the prior art, but rather a “witches-hat” type profile, i.e. a profile that approaches a triangle, with a steadily falling slope from its center to its ends.